[menu-bar] Add _tagNamePrefix to allow customizing button / submenu tag name

[web-padawan]

Motivation

Vaadin Copilot uses vaadin-menu-bar internally for the activation button. For V25, we should update it to use a custom menu-bar e.g. copilot-menu-bar instead, and do not use LumoInjectionMixin there.

Proposed solution

Add _tagNamePrefix to allow customizing tag names for the menu-bar submenu and button:

web-components/packages/menu-bar/src/vaadin-menu-bar-mixin.js

Line 295 in c4a4303

this._subMenuController = new SlotController(this, 'submenu', 'vaadin-menu-bar-submenu', {

web-components/packages/menu-bar/src/vaadin-menu-bar-mixin.js

Line 309 in c4a4303

this._overflowController = new SlotController(this, 'overflow', 'vaadin-menu-bar-button', {